A limitation that exists in all optical systems used in cameras today is the tradeoff between aperture and the depth of field (DOF). An aperture determines the amount of light that enters the camera and the DOF determines the range of distances from the camera that are in focus when the image is captured. The wider the aperture (the more light received) the more limited the DOF.
In many applications the tradeoff between aperture and the DOF becomes apparent. For example, most mobile phones have fixed focus lenses so that only subject within a limited range is in focus. It also places a constraint on the aperture setting of the camera in that the camera must have a relatively small aperture to ensure that as many objects as possible are in focus. This tradeoff reduces the camera's performance in low light situations typically reducing the shutter speed by a factor of 4 or 8.
Further, in low light applications a wide aperture is required, which results in a loss of DOF. In pictures where objects are at different distances from the camera some of the objects will be out of focus even with a focusing lens. Wide aperture lenses require greater precision for optical performance and are therefore expensive.
Techniques to increase the DOF are known in the prior art. One technique referred to as “focus stacking” combines multiple images taken at subsequent points in time and at different focus distances in order to generate a resulting image with a greater depth of field DOF than any of the individual source images. Implementation of focus stacking requires adaptations of the camera electronics and substantial (non-linear) processing and image analyses of relatively large amounts of image data. Moreover, as the focus stacking requires multiple images taken at subsequent moments in time this technique is sensitive to motion blur.
Another approach is described in an article by Green et al., “Multi-aperture photography”, ACM Transactions on Graphics, 26(3), Jul. 2007, pp. 68:1-68:7. In this article the authors propose to increase the FOD using a system that simultaneously captures multiple images with different aperture sizes. The system uses an aperture splitting mirror which splits the aperture in a central disc and a set of concentric rings. The aperture splitting mirror however is complex to fabricate and produces high optical aberrations. Moreover, implementation of such splitting mirror in a camera requires a relative complex optical system which requires precise alignment.
Hence, there is a need in the prior art for a simple and cheap methods and systems for improving the depth of field in an imaging system.
PCT applications with international patent application numbers PCT/EP2009/050502 and PCT/EP2009/060936, which are hereby incorporated by reference, describe ways to extend the depth of field of a fixed focus lens imaging system through use of an optical system which combines both color and infrared imaging techniques. The combined use of an image sensor which is adapted for imaging both in the color and the infrared spectrum and a wavelength selective multi-aperture aperture allows extension of depth of field and increase of the ISO speed for digital cameras with a fixed focus lens in a simple and cost effective way. It requires minor adaptations to known digital imaging systems thereby making this process especially suitable for mass production.
Further, PCT applications with international patent application numbers PCT/EP2010/052151 and PCT/EP2010/052154, which are also hereby incorporated by reference, describe ways to generate depth maps through use of a multi-aperture imaging system.
Although the use of a multi-aperture imaging system provides substantial advantages over known digital imaging systems, there is need in the art for methods and systems which may provide multi-aperture imaging systems with still further enhanced functionality.